Low temperature drawing of metal wires



March 14, 1961 w. c. ELLIS EI'AL LOW TEMPERATURE DRAWING OF METAL WIRES Filed Sept. 12, 1951 BATH AT TEMPERATURE BELOW -40'C Flaz' 555. ii: I

we. 51. L /5 E. s. GRE/NER ATTORNEY P 2,974,778 lcfi Patented Mar. 14, 1

Low TEMPERATURE DRAWING or METAL WIRES William C. Ellis, Maplewood, and Earl S. Greiner, Short Hills, NJ., ass'gnors to Bell Telephone Laboratories, Incofi'porated, New York, N.Y., a corporation of New Yor Filed Sept. 12, 1951, Ser. No. 246,309.

10 Claims. (Cl. 205-21) This invention relates to methods of producing metal wires having improved mechanical properties and to wires so produced.

According to the invention, metal wires are drawn through drawing dies at temperatures below -40 C. and preferably below 60 C. It has been found that the wires produced in this manner from suitable metals have mechanical properties which are substantially better than those of similar wires drawn at conventional wire-drawing temperatures.

One manner in which this procedure can be carried out is shown in the accompanying drawing in which:

Fig. 1 is a plan view of apparatus for carrying out the wire-drawing of the present invention; and

Fig. 2 is a front elevation, in section, of the same apparatus.

In the apparatus shown in Figs. 1 and 2, a spool 1, having wound thereon a wire 2 which is to be drawn to a smaller diameter, is mounted on a spindle 3, suitably supported within a thermally insulated tank 4. A drawing die 5 is mounted on the wall of the tank and is braced by a supporting block 6. The wire 2 is drawn through the die 5 and passes to a take-up reel (not shown). The tank 4 is filled with a low temperature bath 7 which maintains the wire 2 and the die 5 at the temperatures required for the purposes'of the present invention.

Metals having a close-packed atomic structure, namely those having a face centered cubic or a close-packed hexagonal crystalline structure, are sufiiciently ductile to undergo drawing at the low temperatures at which the advantageous results of the present invention are obtained. The term metals, as used in this specification and in the appended claims, is to be understood as including alloys as well as single elemental metals.

Because of their widespread use as electrical conductors, copper and aluminum are particularly advantageous metals to be drawn into wires by the process of the present invention. Alloys of these metals, such as copper-beryllium and certain brasses and bronzes which possess a close-packed atomic structure, can also advantageously be used. Among other suitable metals may be mentioned nickel, titanium, silver, cobalt, palla- -dium and platinum, as well as alloys of these metals with each other and with other face centered" cubic or close-packed hexagonal metals. Similarly, alloys of these metals with metals from other crystalline systems, when these alloys retain the close-packed atomic structure, are suitable for use in the process of the present invention.

Inasmuch as the improvement in mechanical properties which results from drawing at the low temperatures recrystallization but also, at a slower rate, at temperatures somewhat below the temperature of recrystallization. Therefore, in order to achieve the advantageous results of the present invention, it is necessary that the metal which is drawn be one which recrystallizes at a temperature substantially above the temperature range to which the wire is to be subjected in normal use and, in any event, one which recrystallizes at a temperature substatnially above room temperature, or about 25 C.

The degree to which this recovery of the metal occurs, with corresponding loss of improved properties, is dependent upon the purity of the metal. Thus a pure elemental metal shows a tendency to lose its improved properties at a temperature somewhat lower than does the same metal containing a substantial amount of impurities or alloying ingredients. In any event, it is desirable for the purposes of the present invention that the temperature of recrystallization of the metal employed be at least about C., preferably at least about C. and more preferably at least C. Metals, which in their .pure state have lower recrystallization temperatures, may obviously be used in alloys where the alloys have suitably high recrystallization temperatures.

One of the most suitable sources of the low temperature for carrying out the drawing operation is solid carbon dioxide, having a temperature of -78 C. This material can be used by mixing it in comminuted form with a suitable heat transfer liquid to form the low temperature bath 7. Cellosolve acetate has been found to be a suitable liquid for mixing with the solid carbon dioxide. Any other liquid which does not attack the apparatus and which has a sufliciently low freezing point can be employed. By the use of such a low temperature bath, a drawing temperature of about 75 C. can readily be maintained in the metal.

Where lower drawing temperatures are desired, other low temperature baths can be employed. For instance, with liquid nitrogen as the low temperature bath, a drawing temperature of about -l95 C. can be maintained.

The improvement in mechanical properties which is obtained is dependent upon the temperature at which the drawing takes place and upon the total decrease in crosssection accomplished by drawing. The improvement in properties, for any given area reduction, increases as the drawing temperature decreases. No substantial improvement over drawing at room temperature can be obtained by drawing at temperatures higher than about -40 C. Greater improvement, particularly at smaller area reductions, is obtained by drawing at temperatures below 60 C. As indicated above, one of the most practical operating temperatures is 75 C. The lower limit at which drawing can be carried out is set only by the difficulty of providing suitable apparatus.

At any given drawing temperature, the properties improve continuously with the degree of reduction in crosssectional area, usually until an optimum degree of 'area reduction is reached, after which further area reduction may cause the properties to deteriorate somewhat below their optimum value. Drawing is carried out in the usual manner with any convenient area reduction, such as a reduction in area of between 1 percent and 20 percent, with each pass through a drawing die. The metal may be maintained at the low drawing temperature between passes through successive dies or may be allowed to warmto room temperature or any intermediate temperature be tween passes. To achieve the desirable result, it is only necessary that. the low temperature be maintained during actual drawing.

The wire may be passed through successive dies until the total area reduction which is desired is obtained. At all degrees of area reduction, the metal drawn at the low temperatures is substantially superior in physical properties to metal drawn at room temperature to a corresponding area reduction. However, it will ordinarily be desirable that the metal be drawn at low temperatures to an area reduction ofiatleast -percentand preferably at least 20;percent-, in: order toobtain an adequate improvement in physical properties to justify the low temperature drawing process. Substantially greater improvements in physical properties can be obtained by area reductions of at least 40 percent. With most metals, best results areobtained with area reductions between about 45 percent andabout 85 percent. Greater area reductions, particularly with higher drawingtemperatures such as between 60" and 40 C., will often be found desirable.

The elfect of low temperature'drawing upon relatively pure copper will illustrate the improved physical properties to be obtained by the process of the present invention with metals having a close-packed atomic structure and having recrystallization temperatures substantially above room temperature. Specimens of copper wire, 0.05 inch in diameter, in their annealed state possessed an ultimate strength of about 37,000 pounds per square inch' (p.s.i.) a proportional limit of 13,000 p.s.i. and a yield strength (at 0.05 percent strain set) of 16,000 p.s.i. These wires were drawn, with reductions in diameter of 0.002 inch with each pass, through drawing dies until the desired .total area reduction was achieved. With drawing at 195 C. the ultimate strength reached a value of 72,000 p.s.i. at an area reduction of 75 percent, representing a 26 percent higher value than was obtained by similar drawing at 25 C. With an area reduction of 51 percent, an ultimate strength of about 66,000 p.s.i. was obtained representing a value about 24 percent higher than that obtained by similar drawmg at 25 C. At an area reduction of 22 percent, the value of ultimate strength was about 51,000 p.s.i. as compared with about 46,000 p.s.i. obtained by drawing at 25 C. I

Similarly, the yield strength of the wire drawn at --l95 C. reached a value of about 59,000 p.s.i. at an area reduction of 51 percent, representing an improvement of about 26 percent over the value obtained by drawing at 25 C.

Drawing at 75 C. gave an ultimatestrength of 57,000 p.s.i. at an area reduction of 85 percent, representing a 22 percent improvement over that obtained by drawing at 25 C. Ultimate strengths of about 63,000

p.s.i. were obtained at'a 74 percent area-reduction and.

760,000 p.s.i. at a 51 percentarea reduction. Similarly,

at an area reduction of 85 percent, a proportional limit of 52,000 p.s.i. was obtained as compared to a value of 43,500 p.s.i. obtained by drawing at 25 C. These improvements in physical properties by drawing at 75 C; were obtained with only a very small decrease in electrical conductivity below the values obtained by drawing .at 25 C. Therefore, the drawing process of the present invention ofiers a means for'providing copperwire, for

,overhead electrical lines, having substantially'improved. resistance to sagging due to temporary mechanical overload.

Similar improvements in physicalproperties are ob-- smaller openings, while maintaining the temperature of the copper below '60 C. at the point of drawing until the cross-sectional area of the wire has been reduced by at least 20 percent.

2. The process of claim lwherein the copper is drawn at a temperature of about C. and the area reduc:

tion is about percent.

3. The process which comprises drawing a metal wire through a plurality of drawing dies having successively smaller openings, while maintaining the temperature of the wire below 60" C. at the point ofdrawing, until the cross-sectional area of said wire has been reduced by at least 40 percent, the metal of which said wire is formed being one which has a close-packed atomic. structure, which has a recrystallization.temperature. of at least C., and which consists essentially of at least one elemental metal, each said elemental metal being one which has a close-packed atomic structure, 7 I

4. The process which comprises drawing a metal wire through a plurality of drawing dies having successively smaller openings, while maintaining the temperature of the wire below 60 C. at the point of drawing, until the cross-sectional area of the wire has been reduced by at least 20 percent, the metal of which said wire is formed being one which has a close-packed atomic structure, which has a recrystallization temperature of at least 75 C., and the primary constituent of which isan elemental metal having a close-packed atomic structure.

5. The process of claim 4 wherein the metal of which the wire is formed is copper.

6. The process of claim 4 wherein the metal of which the wire is formed is aluminum.

7. The process of claim 4 wherein the metal of which the wire is formed is nickel.

8. The process of claim 4 whereinthe metal of which the wire is formed is titanium. a

9. The process of claim 4 wherein the'metal of which the wire is formed is a copper-beryllium alloy. 7

10. In a wire drawing process in which a wire of a metal is cold drawn, which metal has a close-packed atomic structure and a recrystallization temperature of at least 75 C., and in which process any increase in tensile strength, upon cold drawing, ,is caused by crystal deformation, the improvement which consists in drawing said wire through at least one drawing die while maintaining said wire at a temperature below --40f C. at the point of drawing, wherein the metal of which the wire is composed has a close-packed hexagonal crystalline structure.

pany, Inc., New York,.page141-.

. The Making, Shaping and Treating of Steel, by I. M.

Camp and C. B. Francis, fifth edition, published by Carnegie-Illinois Steel Co.,. Pittsburgh, Pa. pages 608,

609, 610, 611. V V I The. Metallurgy of Iron and Steel, by Bradley Stoughton, third edition, published'by' McGraw-I-lill Book Co.

page'257.

Metals Handbook, 1939edition, pages 12, 78-81, 87-

91, 881, 1714. r r Metals Handbook, 1948 edi'on, pages 259+. 

